Project Summary/ Abstract Differences in both high density lipoprotein (HDL) cholesterol abundance and function likely impact the cardiovascular protective potential of HDL. HDL classically mediates reverse cholesterol transport (RCT) to the liver via processes that include the binding of apolipoprotein A-I (apoA-I) in HDL to scavenger receptor class B, type I (SR-BI) in hepatocytes. Other actions of the HDL/SR-BI tandem may also be protective, such as their capacity to activate endothelial NO production and repair via kinase signaling that uniquely requires the adaptor protein PDZK1. However, we surprisingly discovered that endothelial- specific SR-BI deletion in apoE-/- mice, which have high VLDL/LDL and low HDL, results in decreased atherosclerosis. We have further discovered that this is related to a >80% decline in LDL delivery to the artery wall, revealing the mechanism underlying a critical step in atherogenesis. As such, there may be a dichotomous role for endothelial SR-BI in atherosclerosis, promoting it in the setting of low HDL yet affording atheroprotection in the setting of high HDL. This novel concept and its mechanistic underpinnings will be interrogated in mice and in human endothelial cells. Aim 1 is to compare how endothelial SR-BI influences atherosclerosis in the setting of relatively low vs. high HDL. Atherosclerosis will be evaluated with vs. without endothelial SR-BI and HDL raised via transgenic expression of human apoA-I, and findings will be compared with those made with low HDL. The impact of endothelial SR-BI signaling on atherosclerosis will also be evaluated at low vs. high HDL by deleting endothelial PDZK1. Aim 2 is to determine the roles of endothelial SR-BI and PDZK1 in the putative atheroprotective actions of HDL. To assess HDL's anti-inflammatory actions in vivo, endothelial cell-leukocyte adhesion will be evaluated by intravital microscopy, and HDL, SR-BI and PDZK1 modulation of endothelial genes that influence vascular inflammation will also be interrogated. The roles of endothelial SR-BI and PDZK1 in RCT will be determined in models that query processes in both blood and lymphatic endothelium, and in both microvasculature and arteries. Aim 3 is to determine how endothelial SR-BI governs artery wall LDL deposition. LDL uptake will be studied in cultured endothelium expressing mutant forms of SR-BI with specific loss-of-function for lipid flux, sensing of plasma membrane cholesterol movement, or C-terminal protein interaction. Further studies in culture will evaluate possible roles of other LDL receptors, and co- trafficking of LDL and SR-BI across the endothelium. Aorta LDL uptake will be evaluated in vivo in models testing the role of PDZK1, the impact of elevated HDL, and if pharmacologic intervention targeting SR-BI can decrease LDL deposition. By accomplishing these aims, we will elucidate how vascular health is influenced by endothelial SR-BI, which may be a critical point of intersection of atherosclerosis modulation by HDL and LDL. The new knowledge gained will potentially identify novel therapies for atherosclerosis.